Miniature silicon based thermal vacuum sensor and method of measuring vacuum pressures

Abstract

A miniaturized silicon based thermally controlled vacuum sensor uses thin film resistors on a membrane in a minute measuring chamber to accurately detect vacuum pressures in the range of 760 Torr to 1×10 -5 Torr. The configurations of the measuring chamber and gas diffusion port of the sensor structure insure that heat transfer from the membrane is predominately conductive over the pressure detection range to provide linear output up to 0.1 Torr. A microprocessor is used to control and measure power required to maintain a predetermined temperature differential between a sensing resistive element on the membrane and an ambient temperature sensing element of the sensor base from analog voltage and current values. Pressure detection errors introduced by ambient temperature variations are minimized by measuring power dissipated into the gas. Analog and digital converters for both current and voltage signals use a Σ-Δ conversion method to reject electrical noise by an averaging technique to produce stable signal detection of pressure down to 1×10 -5 Torr. The sensor is thermally stable over an ambient temperature range of 0°-50° C. at pressures between 10 -4 to 760 Torr.

What is claimed is: 
     
       1. A gas pressure sensor for sensing gas pressures in the approximate range of 760 Torr to 1×10 Torr, the pressure sensor comprising; a single piece base having a thickness dimension of at least 100 microns and having a cavity which extends from a bottom surface of the base to a top surface of the base,   an opening in a side wall of the base to the cavity,   a membrane on the top surface of the base dimensioned to cover the cavity,   a first resistive element on the membrane,   a second resistive element on the base,   a cover over the membrane, the cover having a top surface and a bottom surface, the bottom surface of the cap attached to the top surface of the base and spaced from the membrane a distance less than 10 microns,   a controlled voltage source for applying voltages to the first and second resistive elements, and   a measuring circuit for measuring voltage applied to the first and second resistive elements, the voltage measurements being indicative of pressure about the sensor.   
     
     
       2. The pressure sensor of claim 1, wherein the base comprises a single piece of silicon. 
     
     
       3. The pressure sensor of claim 1, wherein internal side walls of the cavity are angled from the bottom surface of the base to the top surface of the base. 
     
     
       4. The pressure sensor of claim 1, wherein a thickness dimension of the base and cavity, measured from the bottom surface of base to the top surface of the base, is approximately four hundred microns. 
     
     
       5. The pressure sensor of claim 1, wherein a thickness dimension of the cap, measured from a top surface of the cap to a bottom surface of the cap, is approximately four hundred microns. 
     
     
       6. The pressure sensor of claim 1, wherein the bottom surface of the cover is parallel to the membrane and is spaced from the membrane a distance in a range of approximately ten microns or less. 
     
     
       7. The pressure sensor of claim 1, wherein the membrane is comprised of silicon nitride. 
     
     
       8. The pressure sensor of claim 1, wherein the first resistive element is comprised of a single continuous pattern of thin film material on the membrane. 
     
     
       9. The pressure sensor of claim 1, wherein the second resistive element is on the top surface of the base. 
     
     
       10. The pressure sensor of claim 1, wherein the membrane has an area in the approximate range of one to two square millimeters. 
     
     
       11. The pressure sensor of claim 1, wherein the membrane has a thickness dimension of approximately one micron. 
     
     
       12. The pressure sensor of claim 1, wherein the first resistive element has a nominal resistive value approximately 200 ohms. 
     
     
       13. The pressure sensor of claim 1, wherein the first resistive element and the second resistive element are formed from a common metallization layer. 
     
     
       14. The pressure sensor of claim 1, wherein the first and second resistive elements are comprised of nickel. 
     
     
       15. The pressure sensor of claim 1, further comprising a passivation layer of silicon nitride over the first resistive element on the membrane and the second resistive element on the base. 
     
     
       16. The pressure sensor of claim 1, wherein the resistance value of the second resistive element is on the order of three to four or more times greater than the resistive value of the first resistive element. 
     
     
       17. The pressure sensor of claim 1, further comprising electrical leads to the first and second resistive elements, the electrical leads mounted on an area of the base adjacent the membrane. 
     
     
       18. The pressure sensor of claim 17, wherein one leg of the first resistive element is connected with one leg of the second resistive element by common connection to one of the electrical leads. 
     
     
       19. The pressure sensor of claim 1, wherein the first resistive element and the second resistive element have approximately equal values of temperature coefficient of resistance. 
     
     
       20. A pressure sensing device for sensing atmospheric pressures, the pressure sensing device comprising, a silicon chip substrate having a gas chamber formed therein, the gas chamber extending through a cross-section of the chip substrate from a bottom surface of the chip substrate to a top surface of the chip substrate,   a top opening of the gas chamber covered by a membrane on a top surface of the chip,   an opening in a side wall of the chip into the gas chamber,   a first resistive element mounted on the membrane,   a second resistive element mounted on the chip and not over the gas chamber,   a cover attached to the chip and covering the membrane and first resistive element, an underside of the cover spaced from the membrane a distance not greater than 10 microns,   electrical leads connected to the first and second resistive elements,   a voltage source connected to a bridge containing the first and second resistive elements, and   a microprocessor for receiving digitally converted analog values of voltages of the bridge applied to the first and second resistive element and for computing power values based upon voltage outputs of the bridge indicative of power required to maintain a predetermined temperature differential between the surface upon which the first resistive element is mounted and the surface upon which the second resistive element is mounted.   
     
     
       21. The pressure sensing device of claim 20, wherein the chip is approximately 2 millimeters square and has a total thickness dimension of approximately 800 microns, and wherein the gas chamber has a thickness dimension of approximately 400 microns. 
     
     
       22. The pressure sensing device of claim 20, wherein the membrane has a surface area of approximately one millimeter square. 
     
     
       23. The pressure sensing device of claim 20 wherein the cover is approximately 400 microns thick. 
     
     
       24. The pressure sensing device of claim 20, wherein the underside of the cover is spaced from the membrane by a depth dimension Δx in the range of approximately five to ten microns. 
     
     
       25. The pressure sensing device of claim 20, wherein an electrical lead connected to the first resistive element is also connected to the second resistive element. 
     
     
       26. The pressure sensing device of claim 20, further comprising an analog-to-digital converter operative to convert mean analog values of voltage and current applied to the first and second resistive elements to digital values. 
     
     
       27. The pressure sensing device of claim 26, wherein the analog-to-digital converter is a Σ-Δ analog-to-digital converter. 
     
     
       28. The pressure sensing device of claim 20, wherein the resistance of the second resistive element is greater than the resistance of the first resistive element. 
     
     
       29. The pressure sensing device of claim 20, wherein the voltage applied to the bridge circuit is controlled by a closed loop control circuit operative to balance voltages on sides of the bridge circuit. 
     
     
       30. The pressure sensing device of claim 24, wherein the closed loop control circuit comprises a field effect transistor input amplifier. 
     
     
       31. The pressure sensing device of claim 20, wherein the bridge circuit further comprises at least one adjustable resistive element in addition to the first and second resistive elements. 
     
     
       32. The pressure sensing device of claim 20, wherein the bridge circuit further comprises at least one resistive element in addition to the first and second resistive elements operative to compensate for thermally induced changes in the temperature coefficients of resistance of the first and second resistive elements. 
     
     
       33. A method of measuring vacuum pressures of a known gas in the approximate pressure range of 760 Torr to 1×10 -5  Tort, comprising the steps of: providing a gas pressure sensor having a silicon chip substrate with a gas chamber which extends through a cross-section of the chip substrate from a top of the chip substrate to a bottom of the chip substrate, a top opening of the gas chamber covered by a membrane attached to a top surface of the chip substrate, a first resistive element mounted on the membrane, a second resistive element mounted on the chip substrate and not over the gas chamber, an opening through a side wall of the chip substrate to the gas chamber, and a cover attached to the chip substrate and covering the membrane and first resistive element, an underside of the cover spaced from the membrane a distance not greater than 10 microns,   applying a voltage to the first resistive element,   applying a voltage to the second resistive element,   controlling the applied voltages to maintain a temperature differential between the membrane and the chip substrate, and   measuring the voltages and currents applied to the first and second resistive elements required to maintain the temperature differential, the measurements being indicative of pressure about the membrane.   
     
     
       34. The method of claim 33, further comprising the step of supplying voltages to the first and second resistive elements through a voltage dividing bridge circuit. 
     
     
       35. The method of claim 34, further comprising the step of controlling the voltages applied to the first and second resistive elements with a closed loop controller operative to balance the voltages on sides of the bridge circuit. 
     
     
       36. The method of claim 33, wherein the step of measuring voltages and currents applied to the first and second resistive elements is performed by converting a mean value of a plurality of voltage and current signals to digital signals and inputting the digital signals to a microprocessor to compute power values applied to the first and second resistive elements. 
     
     
       37. A gas pressure sensing structure for sensing vacuum pressures, the pressure sensing structure comprising: a base having a gas chamber cavity which extends from a top of the base to a bottom of the base,   a membrane covering a top opening to the gas chamber cavity,   a cover attached to the base and covering the membrane, a bottom surface of the cover spaced from a top surface of the membrane a distance not grater than 10 microns, and   a first resistive element disposed at least partially on the membrane,   a second resistive element disposed at least partially on the base,   a voltage source connected to the first and second resistive elements, and   a measuring circuit for measuring voltages and currents applied to the first and second resistive elements, the measuring circuit comprising a bridge circuit having four resistors, the bridge circuit connected to an amplifier, a capacitor and a transistor.